Object Comprising a Steel Part of Metal Construction Consisting of an Area Welded by a High Power Density Beam and Exhibiting an Excellent Toughness in a Molten Area, Method for Producing Said Object

ABSTRACT

The invention discloses an object comprising at least one part made of steel, the composition of which comprises, the contents being expressed by weight, carbon with a content of between 0.005 and 0.27%, manganese between 0.5 and 1.6%, silicon between 0.1 and 0.4%, chromium in a content of less than 2.5%, Mo in a content of less than 1%, optionally one or more elements chosen from nickel, copper, aluminum, niobium, vanadium, titanium, boron, zirconium and nitrogen, the balance being iron and impurities resulting from the smelting, said steel part including at least one zone welded by a high-energy-density beam, characterized in that said welded zone has a microstructure consisting of 60 to 75% self-tempered martensite and, to complement this, 40 to 25% lower bainite, and preferably 60 to 70% self-tempered martensite and, to complement this, 40 to 30% lower bainite.

The present invention relates to metal constructions made of steel welded by a high-energy-density beam, and more particularly to those in which a minimum level of toughness is required in the melted zone so as to obviate the risk of a sudden fracture.

Assembling hot-rolled steel plates and sheets using a high-energy-density beam, such as a laser or an electron beam, has developed particularly over the course of the last twenty years because of certain specific characteristics among which may be mentioned, for example, the very low deformation of the assemblies, the great precision in positioning the beam and the possibility of melting only the amount of material strictly necessary, the appearance of the weld beads requiring no finishing operation, and the possibility of dispensing with stress-relieving treatments.

Among the fields of application of these processes, mention may particularly be made of naval construction, civil engineering equipment, automobiles, and pipes for transporting natural gas and crude oil. Certain applications, in particular those for which the thicknesses and yield strengths involved, or the service stresses, are the greatest, require toughness guarantees so as to obviate the risk of a sudden fracture. Such an eventuality has especially to be taken into account as assemblies formed by a high-energy-density beam may generate defects such as microporosity or shrinkage cavities liable to initiate brittle fracture. The welded zones therefore must have the highest possible toughness in order to obviate any risk.

Various methods have been proposed for achieving a high toughness in the melted zone. Owing to the observation that tough acicular ferrite structures are obtained by nucleation on non-metallic inclusions, the aim has been to introduce this type of particle into the melted zone, for example by means of a prior deposit, as indicated in document JP No. 2000288754. However, this method has several drawbacks. Dispersion of the oxides within the melted zone may not be uniform, resulting in a dispersion in the mechanical properties within this zone. In addition, increasing the fraction of inclusions results in a reduction in upper shelf energy.

For the same purpose, it has also been sought to control the ratio of the aluminum and oxygen contents so as to promote the formation of inclusions favorable to the nucleation of acicular ferrite. Starting from an aluminum-killed steel, this method requires, however, an increase in the oxygen content in the melted zone, leading to the above drawbacks. In addition, with laser welding, the kinetic conditions for the formation of these desirable acicular ferrite structures are not necessarily compatible with the productivity requirements and therefore the cooling rates after welding.

It has also been proposed to increase the toughness of the melted zones by the addition of nickel (the gammagenous element lowering the gamma-alpha transformation temperature) or of a nickel alloy, so that the weight content of this element in the melted zone is between 0.5 and a few percent. Document U.S. Pat. No. 4,527,040 describes, for example, the addition of a nickel alloy in the form of an insert 0.1 mm in thickness before laser assembly. However, this method makes it more difficult to position the beam relative to the joint plane and increases the risk of defects, and possibly corrosion, appearing.

There is therefore a need to have steel assemblies welded by high-energy-density processes that fully guarantee toughness in the melted zone, without an excessive dispersion in the mechanical properties, and to have an economic method of producing these assemblies that does not have the above-mentioned drawbacks.

The objective of the present invention is to provide such welded assemblies and a method for obtaining such assemblies from structural steels.

For this purpose, a first subject of the invention is an object comprising at least one part made of steel, the composition of which comprises, the contents being expressed by weight, carbon with a content of between 0.005 and 0.27%, manganese between 0.5 and 1.6%, silicon between 0.1 and 0.4%, chromium in a content of less than 2.5%, Mo in a content of less than 1%, optionally one or more elements chosen from nickel, copper, aluminum, niobium, vanadium, titanium, boron, zirconium and nitrogen, the balance being iron and impurities resulting from the smelting. The steel part includes at least one zone melted by a high-energy-density beam, with a microstructure consisting of 60 to 75% self-tempered martensite and, to complement this, 40 to 25% lower bainite, and preferably 60 to 70% self-tempered martensite and, to complement this, 40 to 30% lower bainite.

Advantageously, the object is a steel pipe comprising at least one portion having a zone welded in the longitudinal or transverse direction.

Also advantageously, the object consists of at least two hot-rolled or hot-forged plates of steel having the same or different compositions, of the same or different thickness, which are welded together.

Preferably, the high-energy-density beam is a laser beam.

Also preferably, the high-density-energy beam is an electron beam.

Another subject of the invention is a method for producing one of the above objects, which comprises the steps consisting in:

-   -   providing an object comprising at least one part made of steel,         the composition of which comprises, the contents being expressed         by weight, carbon with a content of between 0.005 and 0.27%,         manganese between 0.5 and 1.6%, silicon between 0.1 and 0.4%,         chromium in a content of less than 2.5%, Mo in a content of less         than 1%, optionally one or more elements chosen from nickel,         copper, aluminum, niobium, vanadium, titanium, boron, zirconium         and nitrogen, the balance being iron and impurities resulting         from the smelting;     -   welding, by a high-energy-density process, the steel part to a         steel workpiece of the same or different composition, which may         or may not already form part of the object; and     -   the welding power, the welding speed and the possible         preheating, post-heating or cooling means being chosen in such a         way that a melted zone is obtained with a microstructure         consisting of 60 to 75% self-tempered martensite and, to         complement this, 40 to 25% lower bainite, and preferably 60 to         70% self-tempered martensite and, to complement this, 40 to 30%         lower bainite.

According to another feature of the method, the nitrogen content of the melted zone does not exceed 0.020% and the welding power, the welding rate and the possible preheating, post-heating or cooling means are chosen in such a way that the melted zone cools according to a parameter

$\Delta \; t\; \frac{800}{500}$

such that:

${\Delta \; t_{B}\exp^{{- 0.75}\; {\ln {({\Delta \; {{tB}/\Delta}\; t\; M})}}}} \leq \left( {\Delta \; t\; \frac{800}{500}} \right) \leq {\Delta \; t_{B}\exp^{{- 0.6}\; {\ln {({\Delta \; {{tB}/\Delta}\; t\; M})}}}}$

and preferably:

${\Delta \; t_{B}\exp^{{- 0.7}\; {\ln {({\Delta \; {{tB}/\Delta}\; t\; M})}}}} \leq \left( {\Delta \; t\; \frac{800}{500}} \right) \leq {\Delta \; t_{B}\exp^{{- 0.6}\; {\ln {({\Delta \; {{tB}/\Delta}\; t\; M})}}}\Delta \; t\; \frac{800}{500}}$

expressed in seconds denoting the time that elapses between the temperature of 800° C. and the temperature of 500° C. during the cooling after welding of said welded zone,

with: Δt_(B)=exp^((6.2 CE II+0.74)),

Δt_(M)=exp^((10.6 CE I−4.8))

CE _(I)=C+Mn/6+Si/24+Mo/4+Ni/12+Cu/15+(Cr(1−0.16√{square root over (Cr)})/8)+f(B)

CE _(II)=C+Mn/3.6+Cu/20+Ni/9+Cr/5+Mo/4,

with: f(B)=0, if B≦0.0001%

f(B)=(0.03−1.5N) if 0.0001%<B≦0.00025%

f(B)=(0.06−3N) if 0.00025%<B<0.0004%

f(B)=(0.09−4.5N) if B≧0.0004%,

C, Mn, Si, Mo, Ni, Cu, Cr, B and N denote, respectively, the carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron and nitrogen contents, expressed as percentages by weight, of said melted zone.

According to another feature of the method, the welding is carried out homogeneously and autogenously by a laser beam, the nitrogen content of the steel does not exceed 0.020% and the welding power, the welding rate and the possible preheating, post-heating or cooling means are chosen in such a way that the melted zone cools according to a parameter

$\Delta \; t\; \frac{800}{500}$

such that:

${\Delta \; t_{B}\exp^{{- 0.75}\; {\ln {({\Delta \; {{tB}/\Delta}\; t\; M})}}}} \leq \left( {\Delta \; t\; \frac{800}{500}} \right) \leq {\Delta \; t_{B}\exp^{{- 0.6}\; {\ln {({\Delta \; {{tB}/\Delta}\; t\; M})}}}}$

and preferably:

${{\Delta \; t_{B}\exp^{{- 0.7}\; {\ln {({\Delta \; {{tB}/\Delta}\; t\; M})}}}} \leq \left( {\Delta \; t\; \frac{800}{500}} \right) \leq {\Delta \; t_{B}\exp^{{- 0.6}\; {\ln {({\Delta \; {{tB}/\Delta}\; t\; M})}}}\Delta \; t\; \frac{800}{500}}},$

expressed in seconds, denoting the time that elapses between the temperature of 800° C. and the temperature of 500° C. during the cooling after welding of the melted zone,

with: Δt_(B)=exp^((6.2 CE II+0.74)),

Δt_(M)=exp^((10.6 CE I−4.8))

CE _(I)=C+Mn/6+Si/24+Mo/4+Ni/12+Cu/15+(Cr(1−0.16√{square root over (Cr)})/8)+f(B)

CE _(II)=C+Mn/3.6+Cu/20+Ni/9+Cr/5+Mo/4,

with: f(B)=0, if B≦0.0001%

f(B)=(0.03−1.5N) if 0.0001%<B≦0.00025%

f(B)=(0.06−3N) if 0.00025%<B<0.0004%

f(B)=(0.09−4.5N) if B≧0.0004%,

C, Mn, Si, Mo, Ni, Cu, Cr, B and N denote, respectively, the carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron and nitrogen contents, expressed as percentages by weight, of the welded steel.

According to another feature of the method, the welding is carried out autogenously and homogeneously by an electron beam, the nitrogen content of the steel does not exceed 0.022% and the welding power, the welding rate and the possible preheating, post-heating or cooling means are chosen in such a way that the zone melted by the electron beam cools according to a parameter

$\Delta \; t\; \frac{800}{500}$

such that:

${\Delta \; t_{B}\exp^{{- 0.75}\; {\ln {({\Delta \; {{tB}/\Delta}\; t\; M})}}}} \leq \left( {\Delta \; t\; \frac{800}{500}} \right) \leq {\Delta \; t_{B}\exp^{{- 0.6}\; {\ln {({\Delta \; {{tB}/\Delta}\; t\; M})}}}}$

and preferably:

${{\Delta \; t_{B}\exp^{{- 0.7}\; {\ln {({\Delta \; {{tB}/\Delta}\; t\; M})}}}} \leq \left( {\Delta \; t\; \frac{800}{500}} \right) \leq {\Delta \; t_{B}\exp^{{- 0.6}\; {\ln {({\Delta \; {{tB}/\Delta}\; t\; M})}}}\Delta \; t\; \frac{800}{500}}},$

expressed in seconds, denoting the time that elapses between the temperature of 800° C. and the temperature of 500° C. during the cooling after welding of said melted zone,

with: Δt_(B)=exp^((6.2 CE II+0.74)),

Δt_(M)=exp^((10.6 CE I−4.8))

CE _(I)=C+Mn/6.67+Si/24+Mo/4+Ni/12+Cu/15+(Cr(1−0.16√{square root over (Cr)})/8)+f(B)

CE _(II)=C+Mn/4+Cu/20+Ni/9+Cr/5+Mo/4,

with: f(B)=0, if B≦0.0001%

f(B)=(0.03−1.35N) if 0.0001%<B≦0.00025%

f(B)=(0.06−2.7N) if 0.00025%<B<0.0004%

f(B)=(0.09−4.05N) if B≧0.0004%,

C, Mn, Si, Mo, Ni, Cu, Cr, B and N denote, respectively, the carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron and nitrogen contents, expressed as percentages by weight, of the welded steel.

According to one particular embodiment of the invention, the steel part is welded to a steel workpiece having the same or different composition, of the same or different thickness, which may or may not form part of said object, using a metal filler product.

The invention will now be described more precisely, but not limitingly, with reference to the appended figures in which:

FIG. 1 illustrates the comparison between the hardness of the heat-affected zone (or HAZ) and that of the melted zone using laser welding and using electron beam welding of structural steels;

FIG. 2 shows the comparison between the Charpy V transition temperature at the 28 joule level (TK_(28J)) of the heat-affected zone and that of the zone melted using laser welding and electron beam welding of structural steels;

FIG. 3 illustrates a typical variation in ductile-brittle transition temperature and in the hardness in the heat-affected zone of a structural steel as a function of the cooling rate;

FIGS. 4 and 5 illustrate the influence of the amount of self-tempered martensite on the toughness in the melted zone in laser welding and in electron beam welding respectively; and

FIG. 6 indicates the change in nitrogen content in the melted zone compared with that of the base metal during electron beam welding.

In the assemblies obtained by laser welding or by electron beam welding, the welded part consists of two distinct zones:

-   -   the melted zone, which corresponds to a zone that has passed         through the liquid state during welding, that is to say the zone         in which the temperature was above that of the liquidus of the         welded material; and     -   the heat-affected zone or HAZ, which may broadly encompass all         the zones that have undergone an allotropic transformation         during welding. Hereafter, this term HAZ will be reserved for         those parts of the assembly that remain in the solid state when         heated to the highest temperatures during welding, which are the         location of greater austenitic grain coarsening. These zones,         which are very often more critical from the toughness         standpoint, correspond to maximum temperatures of above         1200-1300° C.

In the case of autogenous welding (i.e. with no filler metal) and homogeneous welding (carried out between two parts having an identical chemical composition), it has been demonstrated that, over a wide range of steel compositions for metal construction, with carbon contents ranging from 0.005% C to 0.27% C by weight, a manganese content ranging from 0.5 to 1.6%, an Si content ranging from 0.1 to 0.4%, a Cr content of up to 2.5% and an Mo content up to 1%, the mechanical properties of the melted zone and the HAZ are very similar. Thus, FIG. 1 shows that the hardness in laser welding and the hardness in electron beam welding are very similar in these two zones.

This similarity also applies to the toughness properties, as shown in FIG. 2, which compares the Charpy V transition temperature at the 28 joule level of the heat-affected zone with that of the melted zone for both types of welding using high-energy-density beams. The microstructures of these two zones are also very similar.

In other words, providing that their compositions are similar, the high-energy-density melted zone may be likened to a wide HAZ from the standpoint of the mechanical properties. This indicates that means for improving the toughness in the laser-melted zone can be based on the experience acquired previously in the field of HAZs.

In this regard, FIG. 3 shows a typical example of the variation in the hardness and in the ductile-brittle transition temperature of the HAZ of a structural steel containing 0.04% C and 1.3% Mn as a function of the cooling rate after welding. This rate is characterized here by

$\Delta \; t\; \frac{800}{500}$

which parameter denotes the time that elapses between the temperature passing from 800° C. to 500° C. during cooling after welding. There is a cooling rate range (lying for this steel composition at about

$\left. {{\Delta \; t\; \frac{800}{500}} \approx {1 - {2\mspace{14mu} s}}} \right)$

for which the toughness is optimum. For much more rapid cooling rates, untempered or “fresh” martensite forms, the properties of which are inferior. On the other hand, a reduction in cooling rate results in the formation of upper bainite or coarse ferritic structures, which are also less tough. The microstructures corresponding to the toughness optimum consist partly of self-tempered martensite, the tempering being due to the welding cycle itself, and partly lower bainite. The self-tempered structure is characterized by the presence of fine carbides precipitated in the martensite laths. These structures optimized from the toughness standpoint are located toward the end of the region where martensite appears, that is to say corresponding to the onset of hardness reduction starting from a substantially horizontal “plateau” corresponding to the hardness of martensite, when

$\Delta \; t\; \frac{800}{500}$

increases.

According to the invention, it has been demonstrated, as FIG. 4 shows, that a proportion of self-tempered martensite of between 60 and 75%, combined as a complement with a proportion of lower bainite between 40 and 25%, results in excellent toughness in the laser-melted zone. When the proportion of martensite is more especially between 60 and 70% combined as a complement with a proportion of lower bainite between 40 and 30%, the transition temperature is below −100° C., which means a particularly high level of toughness.

A similar conclusion may be drawn from FIG. 5, relating to electron beam welding trials on structural steels, the carbon content of which is between 0.1 and 0.17%. A 60 to 75% proportion of self-tempered martensite and, as a complement, 40 to 25% lower bainite is therefore particularly favorable for obtaining melted zones of excellent toughness in high-energy-density welding.

For a given steel composition, among the various variables for assemblies produced by high-energy-density welding (welding power, welding rate, possible preheating or post-heating or cooling means), those resulting, in the melted zone, in a martensite proportion of 60 to 75%, preferably 60 to 70%, combined with a suitable complement of lower bainite, will be chosen. The relationships between the cooling rate after welding and the fraction of martensite will now be explained, taking into account the similarity between the HAZ and the melted zone in assemblies produced by high-energy-density welding.

In the region of the heat-affected zones, it is known from the publication “Metal Construction”, April 1987, pp. 217-223 that the proportion of martensite may be given by the following expressions:

${{martensitic}\mspace{14mu} {fraction}\mspace{14mu} f_{M}} = {\log {\frac{\Delta \; t_{B}}{\Delta \; t_{500}^{800}}/\log}\frac{\Delta \; t_{B}}{\Delta \; t_{M}}}$

or, equivalently:

${\Delta \; t\frac{800}{500}} = {\Delta \; t_{B}\exp^{{- f_{M}}{\ln {({\Delta \; {t_{B}/\Delta}\; t_{M}})}}}}$

with:

$\Delta \; t\frac{800}{500}$

=time elapsing between 800° C. and 500° C. during cooling of the welded zone after welding; Δt_(M)=critical cooling time resulting in 100% martensite; Δt_(B)=critical cooling time resulting in 100% bainite; and log and ln denoting logarithms to the base 10 and to the base e, respectively.

This expression applies when:

${\Delta \; t_{M}} \leq {\Delta \; t\; \frac{800}{500}} \leq {\Delta \; {t_{B}.}}$

The critical cooling times are related to the chemical composition through the following expressions:

Δt_(B)=exp^((6.2 CE II+0.74)),

Δt_(M)=exp^((10.6 CE I+4.8))

with:

CE _(I)=C+Mn/6+Si/24+Mo/4+Ni/12+Cu/15+(Cr(1−0.16√{square root over (Cr)})/8)+f(B)

CE _(II)=C+Mn/3.6+Cu/20+Ni/9+Cr/5+Mo/4,

with: f(B)=0, if B≦0.0001%

f(B)=(0.03−1.5N) if 0.0001%<B≦0.00025%

f(B)=(0.06−3N) if 0.00025%<B<0.0004%

f(B)=(0.09−4.5N) if B≧0.0004%,

these expressions assuming that f(B)≧0, that is to say N≦0.020%. C, Mn, Si, Mo, Ni, Cu, Cr, B and N denote, respectively, the carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron and nitrogen contents, expressed as percentages by weight, of the steel.

Now, as shown above, the similarity between the HAZ and the melted zone in homogeneous autogeneous welding with a high-energy-density beam indicates that the above formulations valid for the HAZ are also applicable to the melted zone.

According to the invention, in the melted zone, a martensite content of between 60 and 75%, preferably between 60 and 70%, combined with a complement of lower bainite, results in excellent toughness. This is obtained if the cooling parameter obeys the following expression:

${\Delta \; t_{B}\exp^{{- 0.75}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}} \leq \left( {\Delta \; t\frac{800}{500}} \right) \leq {\Delta \; t_{B}\exp^{{- 0.6}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}}$

and preferably:

${\Delta \; t_{B}\exp^{{- 0.7}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}} \leq \left( {\Delta \; t\frac{800}{500}} \right) \leq {\Delta \; t_{B}\exp^{{- 0.6}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}}$

Depending on the high-energy-density method used, two cases are to be distinguished:

-   -   in the case of homogeneous autogeneous laser welding, the         composition of the melted zone is practically identical to that         of the base metal. The abovementioned expressions, relating to         the elemental composition of the melted zone, also apply to the         composition of the base metal, that is to say to the composition         of the steel from which the assembly is produced; and     -   in the case of homogeneous autogeneous electron-beam welding, a         change in the composition in the melted zone compared with the         base metal is observed. The nitrogen content is on average         lowered by about 10%, as indicated in FIG. 6, the result of the         low partial pressure above the liquid metal. Moreover, an         average reduction of 10% in the initial manganese content is         also observed, this element possessing a high vapor pressure.         From the initial N and Mn contents in the base metal, the N and         Mn contents in the melted zone are equal to 0.9C and 0.9Mn,         respectively. Under these conditions, the above expressions         become:

Δt_(B)=exp^((6.2 CE II+0.74)),

Δt_(M)=exp^((10.6 CE I−4.8))

CE _(I)=C+Mn/6.67+Si/24+Mo/4+Ni/12+Cu/15+(Cr(1−0.16√{square root over (Cr)})/8)+f(B)

CE _(II)=C+Mn/4+Cu/20+Ni/9+Cr/5+Mo/4,

with: f(B)=0, if B≦0.0001%

f(B)=(0.03−1.5N) if 0.0001%<B≦0.00025%

f(B)=(0.06−2.7N) if 0.00025%<B<0.0004%

f(B)=(0.09−4.05N) if B≧0.0004%,

these expressions assuming that f(B)≧0, i.e. N≦0.022%.

C, Mn, Si, Mo, Ni, Cu, Cr, B and N denote, respectively, the carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron and nitrogen contents, expressed as percentages by weight, of the welded steel.

Of course, the invention may also be transposed to the case in which a steel part is welded to another steel workpiece of different composition, taking into account the relative participation of each element in forming the melted zone, that is to say the dilution factor. The same remark also applies to the case of welding with a filler metal product, the composition and the dilution factor of which have to be taken into account so as to determine the composition of the melted zone.

The present invention will now be illustrated by the following example, relating to laser beam welding.

A steel 12 mm in thickness used to manufacture pipes, having a yield strength of greater than 400 MPa, with the following composition: C=0.1%; Mn=1.45%; Si=0.35%; Al=0.030%; Nb=0.040%; N=0.004%, was welded in autogenous mode by laser welding with no filler metal, with parameters chosen in such a way that the cooling rate

$\Delta \; t\frac{800}{500}$

was equal to 1.7 s. Under these conditions, the fraction of self-tempered martensite in the melted zone, calculated from the above expression (for the case of homogeneous autogenous welding) was 68%, very close to that determined by metallographic examination, with 32% lower bainite as complement. These conditions correspond to those of the invention, which are associated with optimum toughness of the melted zone. In fact, the transition temperature, determined from impact tensile tests on notched cylindrical specimens 4 mm in diameter, was −120° C. This responds to excellent toughness and a high brittle fracture strength of pipes manufactured under these laser welding conditions.

Thanks to the invention, structures welded by high-energy-density welding can therefore be produced inexpensively, without requiring costly addition elements. The invention makes it possible to choose the assembly conditions so as to meet the safety requirements in respect of the risk of sudden fracture. 

1-10. (canceled)
 11. An object comprising at least one part made of steel, the composition of which comprises, the contents being expressed by weight, carbon with a content of between 0.005 and 0.27%, manganese between 0.5 and 1.6%, silicon between 0.1 and 0.4%, chromium in a content of less than 2.5%, Mo in a content of less than 1%, optionally one or more elements selected from the group consisting of nickel, copper, aluminum, niobium, vanadium, titanium, boron, zirconium and nitrogen, the balance being iron and impurities resulting from the smelting, said steel part including at least one zone melted by a high-energy-density beam, wherein said melted zone has a microstructure consisting of 60 to 75% self-tempered martensite and, to complement this, 40 to 25% lower bainite.
 12. The object as claimed in claim 11, wherein the object is a steel pipe comprising at least one portion having a zone welded in the longitudinal or transverse direction.
 13. The object as claimed in claim 11, wherein the object consists of at least two hot-rolled or hot-forged plates of steel having the same or different compositions, of the same or different thickness, which are welded together.
 14. The object as claimed in claim 11, wherein said high-energy-density beam is a laser beam.
 15. The object as claimed in claim 11, wherein said high-energy-density beam is an electron beam.
 16. A method of producing an object as claimed in claim 11, wherein said method comprises: providing an object comprising at least one part made of steel, the composition of which comprises, the contents being expressed by weight, carbon with a content of between 0.005 and 0.27%, manganese between 0.5 and 1.6%, silicon between 0.1 and 0.4%, chromium in a content of less than 2.5%, Mo in a content of less than 1%, optionally one or more elements being selected from the group consisting of nickel, copper, aluminum, niobium, vanadium, titanium, boron, zirconium and nitrogen, the balance being iron and impurities resulting from the smelting; welding, by a high-energy-density process, said steel part to a steel workpiece of the same or different composition, which may or may not already form part of said object; the nitrogen content of the welding-melted zone does not exceed 0.020% and the welding power, the welding speed and the possible preheating, post-heating or cooling means are chosen in such a way that said melted zone cools according to a parameter $\left( {\Delta \; t\frac{800}{500}} \right)$ such that: ${\Delta \; t_{B}\exp^{{- 0.75}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}} \leq \left( {\Delta \; t\frac{800}{500}} \right) \leq {\Delta \; t_{B}{\exp^{{- 0.6}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}\left( {\Delta \; t\frac{800}{500}} \right)}}$ expressed in seconds denoting the time that elapses between the temperature of 800° C. and the temperature of 500° C. during the cooling after welding of said welded zone, with: Δt_(B)=exp^((6.2 CE II+0.74)) Δt_(M)=exp^((10.6 CE I−4.8)) CE _(I)=C+Mn/6+Si/24+Mo/4+Ni/12+Cu/15+(Cr(1−0.16√{square root over (Cr)})/8)+f(B) CE _(II)=C+Mn/3.6+Cu/20+Ni/9+Cr/5+Mo/4, with: f(B)=0, if B≦0.0001% f(B)=(0.03−1.5N) if 0.0001%<B≦0.00025% f(B)=(0.06−3N) if 0.00025%<B<0.0004% f(B)=(0.09−4.5N) if B≧0.0004%, C, Mn, Si, Mo, Ni, Cu, Cr, B and N denote, respectively, the carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron and nitrogen contents, expressed as percentages by weight, of said melted zone.
 17. The method as claimed in claim 16, wherein said welding is carried out homogeneously and autogenously by a laser beam, in that the nitrogen content of said steel does not exceed 0.020% and that the welding power, the welding speed and the possible preheating, post-heating or cooling means are chosen in such a way that said melted zone cools according to a parameter $\left( {\Delta \; t\frac{800}{500}} \right)$ such that: ${{\Delta \; t_{B}\exp^{{- 0.75}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}} \leq \left( {\Delta \; t\frac{800}{500}} \right) \leq {\Delta \; t_{B}{\exp^{{- 0.6}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}\left( {\Delta \; t\frac{800}{500}} \right)}}},$ expressed in seconds, denoting the time that elapses between the temperature of 800° C. and the temperature of 500° C. during the cooling after welding of said melted zone, with: Δt_(B)=exp^((6.2 CE II+0.74)) Δt_(M)=exp^((10.6 CE I−4.8)) CE _(I)=C+Mn/6+Si/24+Mo/4+Ni/12+Cu/15+(Cr(1−0.16√{square root over (Cr)})/8)+f(B) CE _(II)=C+Mn/3.6+Cu/20+Ni/9+Cr/5+Mo/4, with: f(B)=0, if B≦0.0001% f(B)=(0.03−1.5N) if 0.0001%<B≦0.00025% f(B)=(0.06−3N) if 0.00025%<B<0.0004% f(B)=(0.09−4.5N) if B≧0.0004%, C, Mn, Si, Mo, Ni, Cu, Cr, B and N denote, respectively, the carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron and nitrogen contents, expressed as percentages by weight, of the welded steel.
 18. The method as claimed in claim 16, wherein said welding is carried out autogeneously and homogeneously by an electron beam, in that the nitrogen content of said steel does not exceed 0.022% and that the welding power, the welding speed and the possible preheating, post-heating or cooling means are chosen in such a way that said zone melted by the electron beam cools according to a parameter $\left( {\Delta \; t\frac{800}{500}} \right)$ such that: ${{\Delta \; t_{B}\exp^{{- 0.75}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}} \leq \left( {\Delta \; t\frac{800}{500}} \right) \leq {\Delta \; t_{B}{\exp^{{- 0.6}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}\left( {\Delta \; t\frac{800}{500}} \right)}}},$ expressed in seconds, denoting the time that elapses between the temperature of 800° C. and the temperature of 500° C. during the cooling after welding of said melted zone, with: Δt_(B)=exp^((6.2 CE II+0.74)) Δt_(M)=exp^((10.6 CE I−4.8)) CE _(I)=C+Mn/6.67+Si/24+Mo/4+Ni/12+Cu/15+(Cr(1−0.16√{square root over (Cr)})/8)+f(B) CE _(II)=C+Mn/4+Cu/20+Ni/9+Cr/5+Mo/4, with: f(B)=0, if B≦0.0001% f(B)=(0.03−1.35N) if 0.0001%<B≦0.00025% f(B)=(0.06−2.7N) if 0.00025%<B<0.0004% f(B)=(0.09−4.05N) if B≧0.0004%, C, Mn, Si, Mo, Ni, Cu, Cr, B and N denote, respectively, the carbon, manganese, silicon, molybdenum, nickel, copper, chromium, boron and nitrogen contents, expressed as percentages by weight, of the welded steel.
 19. The method of production as claimed in claim 16, wherein said steel part is welded to a steel workpiece having the same or different composition, of the same or different thickness, which may or may not form part of said object, using a metal filler product.
 20. The object as claimed in claim 11 wherein the melted zone has a microstructure consisting of 60 to 70% self-tempered martensite and to complement this, 40 to 30% lower bainite.
 21. The method as claimed in claim 16 wherein the parameter Δt800/500 is described according to the following: ${\Delta \; t_{B}\exp^{{- 0.7}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}} \leq \left( {\Delta \; t\frac{800}{500}} \right) \leq {\Delta \; t_{B}\exp^{{- 0.6}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}}$
 22. The method as claimed in claim 17 wherein the parameter Δt800/500 is described according to the following: ${\Delta \; t_{B}\exp^{{- 0.7}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}} \leq \left( {\Delta \; t\frac{800}{500}} \right) \leq {\Delta \; t_{B}\exp^{{- 0.6}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}}$
 23. The method as claimed in claim 18 wherein the parameter Δt800/500 is described according to the following: ${\Delta \; t_{B}\exp^{{- 0.7}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}} \leq \left( {\Delta \; t\frac{800}{500}} \right) \leq {\Delta \; t_{B}\exp^{{- 0.6}{\ln {({\Delta \; {{tB}/\Delta}\; {tM}})}}}}$ 